The present invention relates to a pneumatic tire having a sidewall rubber, which has improved weatherability and flexing fatigue resistance and is not discolored even if it is used for a long time.
As a sidewall rubber for tires, there has been generally used a rubber (highly unsaturated rubber) with a high content of a double bond, such as a natural rubber, an isoprene rubber and a butadiene rubber. Since these highly unsaturated rubbers have double bonds in the main chain, these are poor in ozone resistance, heat resistance etc., and thus crack may be formed during long-term storage and use. Accordingly, an amine antioxidant and a wax have been mixed therewith for the purpose of preventing crack.
However, there was a problem that the surface of the sidewall rubber was bloomed with the amine antioxidant and the wax during long-term storage or use, and then the surface of the sidewall rubber was discolored.
As a means to solve these problems, use of a rubber excellent in weatherability, such as ethylene propylene diene rubber (EPDM) has been proposed (Japanese Examined Patent Publication No. 52-1761/1977, Japanese Unexamined Patent Publication No. 56-11933/1981, Japanese Unexamined Patent Publication No. 1-297307/1989, Japanese Unexamined Patent Publication No. 8-231773/1996 etc.). However, since EPDM has an ethylene segment, the ethylene segment is crystallized and crack is formed from the crystalline as an origin, when a tire made from EPDM is cooled after it is heated to nearly 100xc2x0 C. in a use thereof.
Accordingly, the object of the present invention is to provide a pneumatic tire having a sidewall rubber which is excellent in weatherability and flexing fatigue resistance, is not discolored and does not form crack from crystalline.
The present invention relates to a pneumatic tire comprising a pair of bead cores, a toroidal carcass fixed by turning down both ends thereof at the bead core, a belt layer arranged in the peripheral direction outside of the crown of the carcass, a tread arranged outside of the belt layer, and a sidewall rubber arranged outside of the carcass,
wherein the sidewall rubber has a two-layer structure comprising an inner layer rubber adjacent to the carcass and an outer layer rubber arranged outside thereof, and the outer layer rubber comprises 40 to 90% by weight of at least one diene rubber selected from a group consisting of a natural rubber, an isoprene rubber, a styrene-butadiene rubber and a butadiene rubber, and 10 to 60% by weight of a rubber obtained by halogenating a copolymer of an iso-monoolefin having 4 to 7 carbon atoms and p-alkylstyrene, as a rubber component.
In the above-mentioned pneumatic tire, a softener for a rubber is preferably mixed with the outer layer rubber.
In the above-mentioned pneumatic tire, the relationship between the JIS A hardness (HSO) of the outer layer rubber and the JIS A hardness (HSI) of the inner layer rubber preferably satisfies the following equation (1):
HSIxe2x88x9210xe2x89xa6HSOxe2x89xa6HSIxe2x88x923xe2x80x83xe2x80x83(1) 
In the above-mentioned pneumatic tire, it is preferable that the diene rubber in the outer layer rubber comprises a natural rubber or an isoprene rubber and a butadiene rubber, and an amount of the butadiene rubber based on the natural rubber or the isoprene rubber is more than 1.
In the above-mentioned pneumatic tire, the thickness of the outer layer rubber is preferably 0.6 to 2.0 mm.
In the above-mentioned pneumatic tire, the thickness of the inner layer rubber is preferably 2 to 8 mm.